Flat Panel Displays (FPDs) have become one of the most important electronic applications due to the progress of technology. The FPDs have been applied to various fields, such as TVs, outdoor panels, display screens of electronic instruments and electronic watches etc. The optoelectronic industry has made great effort in research and development, the FPDs has evolved from the early Cathode Ray Tubes (CRTs) to the now popular Liquid Crystal Displays (LCDs). Furthermore, a type of FPDs called Organic Light Emitting Diodes (OLEDs) have promising future due to advantages such as self-illuminating, high contrast, high luminance, low driving voltage and compactness etc.
In addition, although Thin-Film Transistor-LCDs (TFT-LCDs) driven by thin-film transistors based on glass substrates are currently a popular choice for many, “soft” substrates such as plastic and flexible substrates which compared to glass substrates are lighter and less fragile have become the focus of the next generation applications.
Currently, flexible substrates mainly use plastic substrates made of Polyethersulfone (PES), which has a glass transition temperature of about 200˜220° C., and thermal expansion coefficient of about 50˜60 ppm/° C. However, since flexible substrates tend to change with temperature, expansion of the plastic substrates may occur due to high temperature cycles in the display manufacturing processes, such that the pitch between electrodes thereon may become too large to allow effective alignment of the electrodes of the flexible substrates with electrodes of a flexible circuit board for electrical connections, creating problems such as electrical failure or short circuit.
Referring to FIG. 1, the problem of the pitch between neighboring electrodes 100 on a traditional flexible substrate 10 becoming too large or too small due to temperature variation is illustrated. As a result, the flexible substrate 10 is thermally deformed, such that there is no sufficient area to interface electrodes 110 on a flexible circuit board 11 with the electrodes 100 of the flexible substrate 10 causing electrical failure or more than one electrodes 110 on the flexible circuit board 11 overlap the same electrode on flexible substrate 10 causing electrical short circuit.
Many have tried to find a solution to the problem of thermal stress or deformation due to different thermal expansions of various materials. For example, U.S. Pat. No. 5,644,373 discloses a LCD device that has a pair of substrate mutually separated from each other by a predetermined distance, so that a liquid crystal can be disposed between the substrates, wherein the substrate is a different material with thermal expansion difference within positive or negative 50% to avoid the problem of misalignment. However, this method uses a glass substrate and does not address the problem of thermal stress and expansion of flexible substrates.
Moreover, U.S. Pat. No. 6,489,573 discloses an electrode bonding structure that reduces the effect of thermal expansion in bonding process of the flexible circuit board. The electrode bonding structure comprises a substrate, a circuit board and an Anisotropic Conductive Film (ACF). A dielectric layer and indenting pads are formed on the surface of the substrate. The inner surface of the indenting pads is lower than the surface of the dielectric layer of the substrate by a depth H3. The circuit board is parallel to the substrate with a circuit dielectric layer and bump pads formed thereon. The bump pads are higher than surface of the circuit dielectric layer by about H1. The ACF with a thickness of H2 is disposed between the substrate and the circuit board. In bonding, the bump pads are correspondingly coupled to the indenting pads, and H1≧(H2+H3), thereby reducing the effect of thermal expansion in bonding the circuit board.
Nonetheless, the above technique is only applied to the bonding between a glass substrate and a flexible circuit board; it does not address the thermal expansion in bonding a flexible substrate and a flexible circuit board.
Another approach commonly used in this field is to estimate the pitch of the electrodes after thermal deformation and make flexible circuit board with corresponding electrode pitch accordingly. However, such a method has a complicated manufacturing process and higher production cost.
Moreover, during bonding of flexible substrates and circuit boards, if the manufacturing machine is not properly controlled or faulty, misalignment may occur between the substrates and the circuit boards. As a result, the whole batch may have to be discarded, increasing the production cost.
Therefore, there is a need for a flexible circuit substrate and the method for utilizing a packaging of the flexible circuit substrate that enhance reliability in the alignment process and effectively solves the problem of thermal deformation of the flexible substrates as described above.